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| United States Patent |
5,242,984
|
|
Dillman
, et al.
|
September 7, 1993
|
Sequentially polymerized styrene-isoprene-styrene block copolymer
adhesive composition
Abstract
A predominantly linear sequentially polymerized styrene-isoprene-styrene
block copolymer composition having a styrene content of from about 17% to
about 25% and a diblock content of less than 4% of linear polymeric blocks
for use in adhesives.
| Inventors:
|
Dillman; Steven H. (Houston, TX);
Southwick; Jeffrey G. (Houston, TX)
|
| Assignee:
|
Shell Oil Company (Houston, TX)
|
| Appl. No.:
|
737118 |
| Filed:
|
July 29, 1991 |
| Current U.S. Class: |
525/314; 524/270; 524/271; 525/98 |
| Intern'l Class: |
C08F 297/02; C08L 009/06; C08L 053/02 |
| Field of Search: |
525/314,98
|
References Cited
U.S. Patent Documents
| 3231635 | Jan., 1966 | Holden et al. | 525/271.
|
| 3239478 | Mar., 1966 | Harlan, Jr. | 525/316.
|
| 4822653 | Apr., 1989 | Kauffman et al. | 525/98.
|
| 4833193 | May., 1989 | Sieverding | 524/486.
|
| 4868057 | Sep., 1989 | Himes | 428/412.
|
| 5118762 | Jun., 1992 | Chin | 525/314.
|
| Foreign Patent Documents |
| 0330088 | Feb., 1989 | EP.
| |
| 1213380 | Feb., 1988 | JP.
| |
| 1213380 | Aug., 1989 | JP.
| |
Other References
Promotional Material published by Kuraray Company, Ltd., Dec. 5, 1988.
New Styrene Block Copolymers for Tape and Label Use, by F. Jagisch and J.
Tancrede, presented at PSTC Tech. Sem. Proceedings, May 1990.
|
Primary Examiner: Seidleck; James J.
Assistant Examiner: Warzel; M. L.
Attorney, Agent or Firm: Haas; Donald F.
Claims
We claim:
1. A linear sequentially polymerized styrene-isoprene-styrene block
copolymer which, when incorporated into an adhesive formulation produces
higher holding power to steel than coupled and difunctional initiated
styrene-isoprene-styrene block copolymers, and which has a styrene content
of from about 17% to about 20% and a diblock content of less than 4%.
2. An adhesive comprising the block copolymer of claim 1 and from 20 to 400
parts by weight per 100 parts styrene-isoprene-styrene block copolymer of
a tackifying resin which is compatible with the isoprene block.
3. The copolymer of claim 1 wherein the diblock content is less than 2%.
4. An adhesive comprising the block copolymer of claim 3 and from 20 to 400
parts by weight per 100 parts styrene-isoprene-styrene block copolymer of
a tackifying resin which is compatible with the isoprene block.
Description
BACKGROUND OF THE INVENTION
This invention relates to a novel block copolymer composition for use in
hot melt adhesives. More particularly, it relates to predominantly linear
high triblock content styrene-isoprene-styrene block copolymer
compositions comprised of linear polymeric blocks and adhesives made using
such compositions.
It is known that a block copolymer can be obtained by an anionic
copolymerization of a conjugated diene compound and an alkenyl arene
compound by using an organic alkali metal initiator. Block copolymers have
been produced which comprise primarily those having a general structure
A--B and A--B--A
wherein the polymer blocks A comprise thermoplastic polymer blocks of
alkenyl arenes such as polystyrene, while block B is a polymer block of a
conjugated diene such as polyisoprene. The proportion of the thermoplastic
blocks to the elastomeric polymer block and the relative molecular weights
of each of these blocks is balanced to obtain a rubber having unique
performance characteristics. When the content of the alkenyl arene is
small, the produced block copolymer is a so-called thermoplastic rubber.
In such a rubber, the blocks A are thermodynamically incompatible with the
blocks B resulting in a rubber consisting of two phases--a continuous
elastomeric phase (blocks B) and a basically discontinuous hard,
glass-like plastic phase (blocks A) called domains. Since the A--B--A
block copolymers have two A blocks separated by a B block, domain
formation results in effectively locking the B blocks and their inherent
entanglements in place by the A blocks and forming a network structure.
These domains act as physical crosslinks anchoring the ends of many block
copolymer chains. Such a phenomena allows A--B--A rubber to behave like a
conventionally vulcanized rubber in the unvulcanized state and is
applicable for various uses. For example, these network forming polymers
are applicable for uses such as in adhesive formulations; as moldings of
shoe soles, etc; impact modifier for polystyrene resins and engineering
thermoplastics; modification of asphalt; etc.
In the prior art, such as that exemplified by U.S. Pat. Nos. 3,595,941 and
3,468,972, the disclosures of which are herein incorporated by reference,
the effort was always made to select the particular coupling agent or
reaction conditions that resulted in the highest coupling efficiency. High
coupling efficiency is desired herein in order to produce strong adhesive
compositions. However, almost all commercial polymers are substantially
less than 100% coupled, i.e. they contain a substantial amount of diblock,
typically 5 to 20%. Coupling efficiency is defined as the mass of
molecules of coupled polymer divided by the mass of molecules of coupled
polymer plus the mass of molecules of uncoupled polymer. Thus, when
producing an SIS linear polymer, the coupling efficiency is shown by the
following relationship:
##EQU1##
Coupling efficiency can be determined theoretically from the
stoichiometric quantity of coupling agent required for complete coupling
or coupling efficiency can be determined by an analytical method such as
gel permeation chromotography. Typical prior art coupling efficiency is
from about 80% to almost 100%. In U.S. Pat. No. 4,096,203, coupling
efficiency is controlled from about 20% to about 80%, preferably about 30%
to about 70%. Prior art also disclosed how to blend polymers from
processes of differing coupling efficiency. For example, if a 60%
efficiency is desired, then polymers from processes having an 80%
efficiency and a 40% efficiency may be blended together.
SUMMARY OF THE INVENTION
The present invention relates to predominantly linear high triblock content
sequentially polymerized Styrene-Isoprene-Styrene (S-I-S) block copolymer
compositions comprised of linear polymeric blocks, Preferably, the block
copolymer compositions comprise less than 4 percent diblock and have
styrene contents from about 17 to about 25 percent. These compositions can
be used in adhesives and many other end uses.
DETAILED DESCRIPTION OF THE INVENTION
As is well known, polymers containing both aromatic and ethylenic
unsaturation can be prepared by copolymerizing one or more polyolefins,
particularly a diolefin, in this case isoprene, with one or more alkenyl
aromatic hydrocarbon monomers, in this case styrene. The copolymers may,
of course, be random, tapered, block or a combination of these, in this
case block. The blocks in the copolymers of this invention are linear.
Polymers containing ethylenic unsaturation or both aromatic and ethylenic
unsaturation may be prepared using free-radical, cationic and anionic
initiators or polymerization catalysts. Such polymers may be prepared
using bulk, solution or emulsion techniques. In any case, the polymer
containing at least ethylenic unsaturation will, generally, be recovered
as a solid such as a crumb, a powder, a pellet or the like. Polymers
containing ethylenic unsaturation and polymers containing both aromatic
and ethylenic unsaturation are, of course, available commercially from
several suppliers.
Polymers of conjugated diolefins and copolymers of one or more conjugated
diolefins and one or more alkenyl aromatic hydrocarbon monomers such as
predominantly linear S-I-S block copolymers are frequently prepared in
solution using anionic polymerization techniques, In general, when
solution anionic techniques are used, these S-I-S block copolymers are
prepared by contacting the monomers to be polymerized simultaneously or
sequentially with an organoalkali metal compound in a suitable solvent at
a temperature within the range from about -150.degree. C. to about
300.degree. C., preferably at a temperature within the range from about
0.degree. C. to about 100.degree. C. Particularly effective anionic
polymerization initiators are organolithium compounds having the general
formula:
RLi.sub.n
Wherein:
R is an aliphatic, cycloalipahtic, aromatic or alkyl-substituted aromatic
hydrocarbon radical having from 1 to about 20 carbon atoms; and n is an
integer of 1 to 4.
In general, any of the solvents known in the prior art to be useful in the
preparation of such polymers may be used. Suitable solvents, then, include
straight- and branched-chain hydrocarbons such as pentane, hexane,
heptane, octane and the like, as well as, alkyl-substituted derivatives
thereof; cycloaliphatic hydrocarbons such as cyclopentane, cyclohexane,
cycloheptane and the like, as well as, alkyl-substituted derivatives
thereof; aromatic and alkyl-substituted aromatic hydrocarbons such as
benzene, naphthalene, toluene, xylene and the like; hydrogenated aromatic
hydrocarbons such as tetralin, decalin and the like; linear and cyclic
ethers such as methyl ether, methyl ethyl ether, tetrahydrofuran and the
like.
The concentration of the initiator can be regulated to control the
molecular weight of the overall composition and of the polystyrene blocks.
Generally, the initiator concentration is in the range of about 0.25 to
about 50 millimoles per 100 grams of monomer. The ratio of the initiator
to the monomer determines the block size, i.e. the higher the ratio of
initiator to monomer the smaller the molecular weight of the block.
Methods of controlling the molecular weights of the blocks and the overall
polymer are quite well known. For instance, such are disclosed in U.S.
Pat. Nos. 3,149,182, which states that the amount of monomer can be kept
constant and different molecular weights can be achieved by changing the
amount of catalyst of the amount of catalyst can be kept constant and
different molecular weights can be achieved by varying the amount of the
monomer, and in U.S. Pat. No. 3,231,635, the disclosures of which are
herein incorporated by reference, and many others. A typical block
copolymer composition within the scope of the present invention, having a
polystyrene block molecular weight of around 11,100, a polystyrene content
of 18% and an overall GPC molecular weight of about 168,000 is prepared by
sequentially introducing styrene, isoprene, and styrene into the reactor,
with sec-butyl lithium as initiator. As inert solvent is sued. First,
styrene is polymerized at a monomer to initiator molar ratio of 120 to 1
and the isoprene polymerized at a monomer to initiator molar ratio of
1,500 to 1 and finally styrene again under the same conditions as before.
The temperature may range from about -60.degree. F. to about 300.degree.
F. but preferable is in the range of about 140.degree. F. to about
180.degree. F. to balance rate of reaction with the possibility of thermal
termination. This process offers the advantage of leaving no measurable
residual diblock.
Following the sequential polymerization, the product is terminated such as
by the addition of a protic terminating agent, e.g. water, alcohol or
other reagents or with hydrogen, for the purpose of removing the lithium
radical forming the nucleus for the condensed polymer product. The product
is then recovered such as by coagulation utilizing hot water or steam or
both. The polymers are not hydrogenated.
It is essential to the performance of the present invention that the S-I-S
block copolymers used herein contain more than 96% triblock i.e. they must
contain very little diblock, preferably less than 2%. We have found that
using a high triblock content SIS block copolymer allows one to achieve
greater holding power to steel than when polymers with a substantial
amount of diblock are used in an adhesive formulation or when low diblock
content polymers made by another process are used.
The styrene content of the block copolymers used herein should be between
about 17% and about 25%. A lower styrene content would produce a polymer
that would not have improved holding power. A styrene content in excess of
about 25% would likely result in relatively poor tack and thus be
unacceptable for use in an adhesive.
It is necessary to add an adhesion promoting or tackifying resin that is
compatible with the elastomeric isoprene block. A common tackifying resin
is a diene-olefin copolymer of piperylene and 2-methyl-2-butene having a
softening point of about 95.degree. C. This resin is available
commercially under the tradename Wingtack 95 and is prepared by the
cationic polymerization of 60% piperylene, 10% isoprene, 5%
cyclopentadiene, 15% 2-methyl-2-butene and about 10% dimer, as taught in
U.S. Pat. No. 3,577,398 incorporated by reference. Other tackifying resins
of the same general type may be employed in which the resinous copolymer
comprises 20-80 weight percent of piperylene and 80-20 weight percent of
2-methyl-2-butene. The resins normally have softening points (ring and
ball) between about 80.degree. C. and about 115.degree. C.
Other adhesion promoting resins which are also useful in the compositions
of this invention include hydrogenated rosins, esters of rosins,
polyterpenes, terpenephenol resins and polymerized mixed olefins. To
obtain good thermo-oxidative and color stability, it is preferred that the
tackifying resin be a saturated resin, e.g., a hydrogenated
dicylopentadiene resin such as Escorez.RTM. 5000 series resin made by
Exxon or a hydrogenated polystyrene or polyalphamethylstyrene resin such
as Regalrez.RTM. resin made by Hercules.
The amount of adhesion promoting resin employed varied from about 20 to
about 400 parts by weight per hundred parts rubber (phr), preferably
between about 100 to about 300 phr.
The selection of the particular tackifying agent is, in large part,
dependent upon the specific block copolymer employed in the respective
adhesive composition. In the manufacture of disposable articles such as
diapers, sanitary napkins and bed pads, there is the additional
consideration of having a substantially white or clear adhesive
composition.
The adhesive composition of the instant invention may contain plasticizers,
such as rubber extending plasticizers, or compounding oils or liquid
resins. Rubber compounding oils are well-known in the art and include both
high saturates content oils and high aromatics content oils. Preferred
plasticizers are low aromatic content oils, e.g. Tufflo.RTM. 6056 oil made
by Arco and Shellflex.RTM. 371 oil made by Shell. The amounts of rubber
compounding oil employed in the invention composition can vary from about
0 to about 100 phr, and preferably between about 0 to about 60 phr.
Optional components of the present invention are stabilizers which inhibit
or retard heat degradation, oxidation, skin formation and color formation.
Stabilizers are typically added to the commercially available compounds in
order to protect the polymers against heat degradation and oxidation
during the preparation, use and high temperature storage of the adhesive
composition.
Additional stabilizers known in the art may also be incorporated into the
adhesive composition. These may be for protection during the life of the
disposable article against, for example, oxygen, ozone and ultraviolet
radiation. However, these additional stabilizers should be compatible with
the essential stabilizers mentioned herein-above and their intended
function as taught herein.
The adhesive compositions of the present invention are typically prepared
by blending the components at an elevated temperature, preferably between
about 130.degree. C. and about 200.degree. C., until a homogeneous blend
is obtained, usually less than three (3) hours. Various methods of
blending are known to the art and any method that produces a homogeneous
blend is satisfactory.
The resultant adhesives may then preferably be used in a wide variety of
pressure sensitive and product assembly applications. Examples include
tapes, labels, diapers, sanitary napkins and decals.
In the following examples, the percent triblock was determined by Gel
Permeation Chromatography. The RVTD viscosity was measured in centipoise
(cps) by ASTM D-3236 using a Brookfield RVTD Thermocell viscometer with a
number 29 spindle. The SAFT was measured by 1".times.1"Mylar to Mylar lap
joint with a 1 kg weight. SAFT measures the temperature at which the lap
shear assembly fails under load. The molecular weights were determined by
gel permeation chromatography as styrene equivalent molecular weight. The
polystyrene content was determined by nuclear magnetic resonance
spectroscopy. Rolling Ball Tack is the distance a steel ball rolls on the
adhesive film with a standard initial velocity (PSTC test no. 6). Small
numbers indicate aggressive tack. Holding Power is the time required to
pull a standard area (1/2 in. .times.1/2 in.) of tape from a standard test
surface (steel, Kraft paper) under a standard load (2 kg), in shear at
2.degree. antipeel (Pressure Sensitive Tape Council Method No. 7). 180
degree peel was determined by Pressure Sensitive Tape Council Method no.
1. Polyken probe tack was determined by ASTM D-2979. Loop tack was
determined using TLMI loop tack tester.
EXAMPLES
A number of polymer samples were used to make adhesive formulations and
tested for performance. In each case, the adhesvie formulation comprised
100 parts of the polymer of interest, 125 parts of ESCOREZ.RTM. 1310
hydrocarbon resin and 20 parts of SHELLFLEX.RTM. 371 oil. The composition
was dissolved in sufficient toluene to form a 40% solids by weight
solution. The solution was then drawn over a 1 mil thick polyester film
and dried to form a 1.5 mil adhesive layer. Drying was accomplished by
evaporating the solvent in a hood for 1 hour followed by 4 hours in a
vacuum oven at 104.degree. F. The samples were then placed in a room at
74.degree. F. and 50% relative humidity for 16 hours prior to testing.
Testing was performed according to the procedure as set forth above.
The polymers used herein are described in Table 1 below. Polymer A was
produced by the well-known coupling process such as described in U.S. Pat.
No. 4,096,203 which is herein incorporated by reference. An alkyllithium
compound was reacted with styrene in an inert solvent. When the reaction
has essentially reached completion, isoprene was introduced and allowed to
react to form a diblock polymer. Finally, a difunctional coupling agent
was introduced resulting in a linear triblock polymer structure. This
process allows control of coupling efficiency but it suffers from the
disadvantage that typically from 5 to 20% of uncoupled diblock remains in
the polymer. Polymer A contained only about 82% triblock. It is believed
that Polymer B was made by a process known as difunctional initiation.
This involves the use of a difunctional organolithium compound as an
initiator. The difunctional organolithium is reacted with an isoprene
monomer to form a polyisoprene with two living ends, Li-I-Li. Styrene is
then introduced to form Li-SIS-Li. Finally, the polymer is terminated with
a protic terminating agent. This process apparently produces a polymer
with no significant amount of diblock. However, it is likely somewhat more
difficult to control than the sequential process used herein.
The other polymer samples listed in Table 1 were made by the sequential
polymerization process of the present invention. In this process, half of
the total styrene to be included in the polymer is reacted with an
alkyllithium (sec-butyl lithium) compound in an inert solvent
(cyclohexane) to form polystyryllithium. The product is then reacted with
isoprene to form a living diblock polymer having the structure SI-Li. At
this point, the remaining styrene is added to the product to form a living
triblock of the form SIS-Li. Finally, the polymer is terminated with a
protic terminating agent (methanol). The polymerization reactions were
performed at 120.degree. F. to 200.degree. F. There was no measurable
diblock content in any of the four polymers made by this process but, for
reasons discussed below, there may have been some diblock in RP6407-89. It
is not possible to get a quantitative measure of how much diblock is
present in that polymer since current GPC technology is incapable of
resolving it from the triblock at such low levels.
TABLE 1
______________________________________
Triblock Content
Polymer Molecular Weight
PSC (%) by GPC (%)
______________________________________
Polymer A
218,000 14.8 82
Polymer B
181,000 17.9 100
PP3811 168,000 17.8 100
RP6407-87
177,000 16.8 100
RP6407-89
186,000 17.0 100
RP6407-1020
167,000 18.3 100
______________________________________
Table 2 shows data comparing PP3811 with Polymers A and B. The critical
differences are in holding power. PP3811 provides twice the holding power
to steel of Polymer B and over four times the holding power to steel of
Polymer A. The other properties are roughly equivalent for the three
polymers.
TABLE 2
______________________________________
A B PP3811
______________________________________
Rolling Ball Tack (cm)
0.6 0.8 0.8
Polyken Probe Tack (kg)
1.2 1.1 1.0
Loop Tack (oz/in) 134 130 117
180.degree. Peel (pli)
6.2 6.0 4.6
Holding Power/Kraft (min)
330 220 740
Holding Power/Steel (min)
260 530 1150
SAFT Kraft (.degree.C.)
56 47 53
SAFT Mylar (.degree.C.)
89 90 91
______________________________________
Table 3 shows data for five polymers, the three mentioned above, RP6407-89
and PP3811 with 4% of a styrene-isoprene diblock added to it. All three of
the polymers made according to the present invention, PP3811, PP3811 plus
4% diblock and RP6407-89, have higher holding power to steel than Polymers
A and B. Even so, it can be seen that adding 4% diblock to PP3811
drastically decreases its holding power to steel. The holding power to
steel of RP6407-89 is much lower than that of PP3811, probably due to the
lower styrene content and a higher level of dieout at the interface
between the rubber block and the second styrene block which produces a
high molecular weight diblock. As stated above, this cannot be measured.
TABLE 3
______________________________________
PP3811
+4% RP6407
PP3811
Db B -89 A
______________________________________
Rolling Ball Tack (cm)
3.6 11.0 2.0
Polyken Probe Tack
1.1 0.7 1.1
(kg)
Loop Tack (oz/in)
97 87 102
180.degree. Peel (pli)
4.8 3.6 5.3
Holding Power/Kraft
310 260 630 480 220
(min)
Holding Power/Steel
1310 580 540 740 290
(min)
SAFT Kraft (.degree.C.)
62 58 65
SAFT Mylar (.degree.C.)
93 92 89
______________________________________
Table 4 compares Polymer A with three polymers made by the present
invention, PP3811, RP6407-87 and RP6407-89. Again, it can be seen that the
holding power to steel of all three of the invention samples is higher
than that of Polymer A. The holding power of RP6407-87 is much lower than
the other two. It is believed that this is due to the lower styrene
content of RP6407-87.
TABLE 4
______________________________________
RP6407 RP6407
PP3811
-87 -89 A
______________________________________
Rolling Ball Tack (cm)
1.6 1.2 2.4 1.0
Polyken Probe Tack (kg)
0.9 1.1 1.5 1.5
Loop Tack (oz/in)
100 111 112 117
180.degree. Peel (pli)
5.5 5.7 5.4 5.3
Holding Power/Kraft (min)
390 300 260 240
Holding Power/Steel (min)
2560 370 720 340
SAFT Kraft (.degree.C.)
58 63 61 55
SAFT Mylar (.degree.C.)
93 90 90 92
______________________________________
Table 5 compares PP3811, RP6407-87, RP6407-1020 and Polymer A. RP6407-1020
is seen to provide holding power to steel equal to that of PP3811. The
remaining properties are roughly equal to those of Polymer A.
TABLE 5
______________________________________
RP6407 RP6407
PP3811
-87 1020 A
______________________________________
Rolling Ball Tack (cm) 0.9 0.7
Polyken Probe Tack (kg) 1.4 1.4
Loop Tack (oz/in) 66 74
180.degree. Peel (pli) 6.5 7.0
Holding Power/Kraft (min)
520 530 530 200
Holding Power/Steel (min)
1010 480 1140 230
SAFT Kraft (.degree.C.) 50 53
SAFT Mylar (.degree.C.) 93 90
______________________________________
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